Using Polymer Hydrogels for Glyphosate Sequestration from Aqueous

Subscriber access provided by University of Sunderland

Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Using polymer hydrogels for glyphosate sequestration from aqueous solutions: Molecular theory study of adsorption to polyallylamine films Néstor Ariel Pérez-Chávez, Alberto Gustavo Albesa, and Gabriel S. Longo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02727 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Using polymer hydrogels for glyphosate sequestration from aqueous solutions: Molecular theory study of adsorption to polyallylamine films Néstor A. Pérez-Chávez, Alberto G. Albesa, and Gabriel S. Longo∗ Instituto de Investigaciones Fisicoqu´ımicas Teóricas y Aplicadas (INIFTA), UNLP-CONICET, Diag.113 y 64 S/N, B1904 La Plata, Argentina E-mail: [email protected]

Abstract A molecular theory has been applied to study the equilibrium conditions of glyphosate and AMPA adsorption from aqueous solutions to hydrogel films of cross-linked polyallylamine (PAH). This theoretical framework allows for describing size, shape, state of charge/protonation, and configurational freedom of all chemical species in the system. Adsorption of glyphosate is a non-monotonic function of the solution pH, which results from the protonation behavior of both adsorbate and adsorbent material. Glyphosate and chloride ions compete for adsorption to neutralize the polymer charge; lowering the solution salt concentration enhances the partition of glyphosate inside the hydrogel film. AMPA adsorption is qualitatively similar to that of glyphosate but orders of magnitude smaller under the same conditions. AMPA is less charged than glyphosate, which unbalances the competition for adsorption with salt counter ions. In mixed solutions, glyphosate presence can significantly hinder AMPA adsorption. A higher pH establishes inside the film than in the bulk solution, which has important implications for the herbicide biodegradation since microbial activity is pH dependent. Thus, PAH

1

ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

hydrogel films can be considered as functional materials that combine glyphosate sequestration and in-situ degradation. In devising these materials, the polymer density is an important variable of design; polymer networks with high density of titratable units can enhance adsorption; this density can also be used to modify the pH inside the material.

Introduction Glyphosate is the world best-selling and most widely applied pesticide. 1,2 In Argentina, glyphosate use continues to increase with 0.2 megatons consumed in 2012, which represents 80% of total herbicide sales in the country. 3 This efficient broad spectrum herbicide, used non-selectively in agriculture, works by blocking a biochemical pathway leading to production of essential amino acids, causing plant death by starvation. 4 Since humans do not produce essential amino acids, glyphosate use is presumed safe. However, its efficiency as a weed killer, its non-toxicity, and the engineering of genetically-modified crops that can resist glyphosate exposure, have, in many cases, led to excessive use in the control of weeds and grasses. Glyphosate has been detected in different soils, groundwaters, surface waters and its bottom sediments, not only in those used for agriculture; 5–7 it has also been found in processed food, drinking water, and pharmaceutical products. 8,9 Therefore, it is nowadays clear that development of means for glyphosate removal is of critical importance. Current methods of glyphosate removal from aqueous solutions include chemical precipitation, 10 advanced oxidation, 11,12 membrane filtration, 13 biodegradation, 14 and physical adsorption. 15–18 Methods involving physical adsorption are generally efficient, low cost and more environmentally friendly, producing no secondary pollution. 15 Different adsorbent materials have been considered for glyphosate sequestration, including resins, 15 graphene oxides, 16 starch, 17 and zeolites. 18 In all those studies, glyphosate adsorption is due to electrostatic attractions with the substrate. These interactions are highly modulated by environmental conditions, such as the pH and salt concentration. Indeed, several experimental studies 2


Page 2 of 28

Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

show the importance of pH and salt concentration in the physical adsorption of glyphosate to different materials. 15,17,18 Therefore, understanding the physical and chemical properties of that govern pesticide adsorption and their coupling with environment composition is essential in the development of materials with optimal contaminant removal/sequestration capacity. In this work, we investigate the adsorption of glyphosate, AMPA and their mixtures to grafted poly(allylamine hydrochloride) (PAH) cross-linked networks. We are particularly interested in the role that environment pH and salt concentration have in determining the adsorption behavior. To this goal, we have applied a molecular-level theory that allows for describing size, shape, state of protonation, and conformations of all the chemical species in the system. Degradation of glyphosate in soils and waters occurs mainly due to bacterial activity. 19,20 In one of the paths for biodegradation, an oxidase breaks down the carbon-nitrogen bond of glyphosate, which produces aminomethylphosphonic acid (AMPA), the most abundant byproduct of this herbicide. 20 However, when the medium pH is not optimal, bacterial activity slows down, which reduces degradation efficiency and results in longer environmental persistence. 20 Our goal is to show that PAH films can combine enhanced glyphosate sequestration with a higher pH environment in the interior of the hydrogel which provides optimal conditions for biodegradation.

Method To study the adsorption of glyphosate (and AMPA) to hydrogel films of grafted cross-linked PAH chains, we apply a molecular-level theory with explicit description of all chemical species that compose the system. In this approach, the protonation state of each of the different titratable units of glyphosate is not assumed but predicted as a result of the local interplay between the free energy cost of protonation/deprotonation, the entropic loss of molecular confinement, the conformational degrees of freedom of the network and the adsorbate, and

3


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1: Left: Scheme representing the adsorption of glyphosate from a salt solution to a surfacegrafted, cross-linked PAH network. The coarse-grained molecular model is illustrated on the right. AMPA is modeled similarly to glyphosate but without including the carboxylate unit.

the electrostatic and steric interactions. This is achieved through the formulation of a general free energy that includes all of these contributions. This molecular theory was originally developed to study adsorption of histidine peptides to grafted polyacid networks, 21 and has been recently extended to investigate protein protonation upon adsorption to such pHresponsive hydrogel films. 22,23 The method is based on an extension of the theory developed by Nap et al. 24 and Gong et al. 25 to consider the behavior of grafted weak polyelectrolyte layers. Next, we outline the theoretical method used in this study, while a full description can be found in our recent work. 23 The system consists of an aqueous solution in contact with a network of cross-linked polyallylamine that is chemically grafted to a planar surface (see Fig. 1). The coordinate z measures the distance from the surface placed at z = 0. The solution is composed of water molecules, hydronium and hydroxyde ions, and a monovalent salt (NaCl), which is assumed to be completely dissociated into chloride and sodium ions. This solution can additionally contain a finite concentration of either glyphosate (GLP), AMPA or both molecules; we will refer to theses species as the adsorbates.

4


Page 4 of 28

Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

The Helmholtz free energy of the system can be expressed as: F = − T Scnf,nw + Fchm,nw − T Smix X + (−T Scnf,i − T Str,i + Fchm,i )

(1)

i

+Ust + Uvdw + Uelect where the first term in the right-hand side of this expression includes the conformational entropy of the network (Scnf,nw ), which is due to the many possible spatial distributions of PAH segments (conformations); T represents system temperature. The second term is the chemical free energy of the network (Fchm,nw ) that accounts for the acid-base equilibrium of PAH segments. The next term includes the translational and mixing entropy of all free species, except the adsorbates, as well as the formation self-energies of these species. In the second line of Eq. (1), the sum runs over glyphosate and/or AMPA if they are present in the solution; for a particular adsorbate (i ∈ {GLP,AMPA}), these terms include its conformational entropy (Scnf,i ), its translational entropy and self-energy (Str,i ), and the chemical free energy (Fchm,i ) that results from the acid-base equilibrium of its titratable units. The third line in Eq. (1) includes the energetic contributions to the free energy, incorporating steric (excluded volume) repulsions (Ust ), van der Waals attractions (Uvdw ) and electrostatic interactions (Uelect ). This system is equilibrium with a bulk solution, which fixes the chemical potentials of all free species. As a result, the proper thermodynamic potential whose minimum yields equilibrium is the semi-grand potential, which is a function of these chemical potentials. The semi-grand potential results from the Legendre transform of the Helmholtz free energy (Eq. (1)). The aforementioned contributions to the free energy can be explicitly written in terms of a few functions: (i) the local densities of all free species; (ii) the local densities of different configurations of the adsorbates; (iii) the probability distribution of network conformations; (iv) the local degrees of protonation of all titratable species including network and adsorbate

5


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

units; and (v) the local electrostatic potential. The next step in this method is the optimization, with respect to each of these functions, of the semi-grand potential. Such procedure allows for expressing functions (i) to (iv) in terms of only two position-dependent interaction potentials: the local osmotic pressure and the electrostatic potential. These local interaction potentials can be numerically calculated through iteratively solving both the Poisson equation and the incompressibility constraint imposed to the fluid. This last restriction assures that, at each position, the available volume is fully occupied by some of the chemical species. After these interaction potentials are determined, any thermodynamic quantity of interest can be derived from the free energy. At this point, the local functions that compose the free energy are known as well, which allows for the calculation of different local and average quantities.

Molecular Model A molecular model of all chemical species must be defined in order to apply this theory and calculate results. Glyphosate is represented using a coarse-grained model where we include the phosphonate, amine and carboxylate groups. The molecular volumes assigned to these coarse-grained units, as well as to other species in the system, are presented in Table 1. To construct this coarse-grained model we have performed density functional theory (DFT) 26 geometry optimizations using Gaussian 03 package; 27 the B3LYP hybrid exchangecorrelation functional 28,29 and the 6-31G(d,p) basis set were used. All geometrical parameters were optimized without constraints. The coarse-grained units are assigned to the position of the Carbon atom in the carboxylate group, the Nitrogen in the amine, and the Phosphorus in the phosphonate group. These positions are kept fixed with respect to the center of mass of the glyphosate molecule according to the DFT optimization of the atomistic structure. The molecule, however, has full rotational and translational freedom. AMPA molecule is represented in the same way, but without including the carboxylate unit. The pKa values of titratable units have been obtained from the literature. 30–33 For 6


Page 6 of 28

Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Table 1: Molecular volumes of the coarse-grained units considered in our model.

species/group H2 O, OH − , H3 O+ Salt Ions (N a+ , Cl− ) phosphonate amine carboxylate allylamine

volume 0.030 nm3 0.033 nm3 0.054 nm3 0.040 nm3 0.056 nm3 0.068 nm3

glyphosate, these values are 0.8 and 5.6 for the diacidic phosphonate group, 2.3 for the carboxylic acid, and 11.0 for the basic amine group; 30,32 for AMPA, we have used 1.8 and 5.4 for the phosphonate and 10.0 for the amine group. 30,32 In the case of the basic allylamine segments of the network, their pKa is 8.5. 31,33 For the self-dissociation of water we use pKw = 14. Using this pKa scheme, Fig. 2 shows the charge of glyphosate and AMPA in dilute solution as a function of pH. The plot also includes the degree of charge of an isolated allylamine monomer under the same conditions. Given that adsorption to the PAH film is expected to be driven by adsorbate-network electrostatic attractions, these curves anticipate a nonmonotonic pH-dependence of such adsorption. At low pH, PAH segments will be strongly positively charged, but glyphosate or AMPA adsorption will bring less net charge to the film than the adsorption of a (smaller) chloride ion. Similarly at high pH, glyphosate and AMPA are more negatively charged, but PAH segments will only be weakly charged. Only for intermediate pH values, where both adsorbent and adsorbate are strongly charged, we expect significant adsorption to occur. The different molecular conformations of the PAH network are also an input needed to evaluate this theory. This network is composed of cross-linked 50-segment long polymer chains, where each segment is a coarse-grained representation of a PAH unit, having 0.5 nm segment length. Most of these chains connect two cross-linking segments, except those topmost chains, which have their solution-side ends free, and some chains that are connected by one of their ends to a surface-grafted segment. Cross-linking units have coordination four,

7


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2: Plot of average glyphosate and AMPA charge in dilute solution as a function of pH (lefthand side y-axis). The right-hand side y-axis corresponds to the degree of protonation of the isolated allylamine monomer under the same conditions.

and the structure has diamond-like topology. 34–37 To generate network conformations, we have performed MD simulations using GROMACS 5.1.2, 38–40 where the network is a periodic molecule composed of 30 cross-linking segments, 2 grafting points, and 64 chains with a total of 3200 PAH units. The supporting surface in the MD simulation box has a 33 nm2 area; periodic boundary conditions are imposed in both the x and y directions. The force field used in this MD simulations has been well described in other works. 34,36,41,42 In order to numerically solve the equations resulting from the molecular theory, the Poisson equation and the incompressibility constraint, space is discretized into 0.5 nm-thick layers parallel to the supporting surface (the x − y plane). The system is assumed to be isotropic in the x and y directions. The aqueous medium has dielectric constant w 0 , where w = 78.5 is the relative dielectric constant of water at room temperature, and 0 is the permittivity of vacuum.

8


Page 8 of 28

Page 9 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Results and Discussion We will first consider the effect of solution pH and salt concentration on the adsorption of glyphosate to PAH films. To quantify such behavior, we define the adsorption as the number of molecules adsorbed per unit area, in excess of the bulk contribution. Namely, Z Γ=

∞

dz ρglp (z) − ρbulk glp

(2)

0

where ρglp (z) and ρbulk glp are the local and bulk (number) density of glyphosate, respectively, such that limz→∞ ρglp (z) = ρbulk glp . Note that Γ includes contributions from adsorbed molecules throughout the whole thickness of the hydrogel film as well as from the polymer-solution interface.

Figure 3: Plots of glyphosate adsorption, Γ, as a function of the bulk solution composition: Panel A shows Γ as a function of the salt concentration for different pH values; panel B shows the adsorption as a function of the pH for various salt concentrations. In both panels, the concentration of the adsorbate in the bulk solution is [glp] = 10 µM.

Figure 3 describes the adsorption of glyphosate as a function of the bulk solution composition. Panel A shows that Γ is a decreasing function of salt concentration, which is a clear signature that adsorption is driven by PAH-glyphosate electrostatic attractions. An increasing salt concentration makes the adsorption of chloride ions more favorable, which results in the screening of these attractions diminishing the capacity of the film to adsorb the

9


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

herbicide. Panel B of Fig. 3 shows that glyphosate adsorption is a non-monotonic function of the solution pH, a result that can be explained by looking at the charge behavior of both species the adsorbate and adsorbent material, displayed in Fig. 2. At low pH, most segments of the polymer network are charged, but glyphosate is mostly in its electro-neutral state; chloride ions will preferentially adsorb as counter-ions under such conditions. At high pH, on the other hand, the situation reverses: while glyphosate is negatively charged, polymer network units are deprotonated (uncharged). As a result, negligible adsorption occurs at both extremes of the pH scale. In the intermediate pH range, where both adsorbent and adsorbate are strongly and oppositely charged, significant adsorption takes place, leading to the non-monotonic curves observed in Figure 3B. The general trends of the behavior predicted in Fig. 3 when changing the solution composition have been experimentally found for glyphosate adsorption to other materials. Guo et al. 17 have recently highlighted the critical importance of pH and NaCl concentration in the adsorption of glyphosate from aqueous solutions to cross-linked amino-starch. Their work shows decreasing adsorption with increasing salinity as well as a non-monotonic pHdependent adsorption with a maximum around pH 6. In addition, Zhou et al. 15 have shown that the adsorption of glyphosate from water to nano-sized microporous exchange resins, modified with copper hydroxide, depends non-monotonically on the pH having a peak around pH 6 − 7. This pH dependence has also been reported upon glyphosate adsorption to zeolites. 18 In these different systems, the non-monotonic behavior can be understood with a slight modification of our previous argument using the pH dependence of the bulk solution charge. However, we will show that such reasoning to understand glyphosate adsorption can be misleading in other situations. In particular because glyphosate deprotonation upon adsorption plays a critical role in defining adsorption to PAH films. In both panels of Fig. 3 maximum adsorption of glyphosate is achieved near pH 5. The net charge (number) of the molecule is approximately −1 at this pH (see Fig. 2), the same as chloride ions. This raises the question of what drives the adsorption behavior observed

10


Page 10 of 28

Page 11 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 4: Plots of different local quantities at pH 5 and two salt concentrations, 1 mM (black curves) and 10 mM (red curves): local polymer volume fraction (A), local glyphosate concentration (B) and net charge (C), and local pH (D), all as a function of the distance to the supporting surface, z. The inset in panel B shows the local concentration of chloride ions at the same conditions. The bulk concentration of glyphosate is [glp] = 10 µM.

11


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

in Fig. 3, since the adsorption of the more abundant ([salt] > [glp]), smaller chloride ion is expected. To address this question Fig. 4 shows different local quantities as a function of the distance to the supporting surface at pH 5 for both low and high salt concentrations. Panel A shows PAH volume fraction under such conditions; this curve is not very sensitive to an increase in the salt concentration, so we only include the case [salt] = 1 mM. We find that the polymer network extends up to h ≈ 130 nm from the surface; below this distance (z < h) that defines the thickness of the hydrogel film, PAH segments occupy roughly 5% of the local volume. Panel B of Fig. 4 shows the local concentration of glyphosate. This graph shows that the behavior described in Fig. 3 is not a surface phenomenon, but the adsorbate partitions at different spatial regions throughout the whole thickness of the hydrogel film. Inside the film, this concentration increases several orders of magnitude respect to the bulk value (10−5 µM). Consistent with the results of Fig. 3, lowering the salt concentration decreases the local concentration of glyphosate inside the film. At 1 mM salt, glyphosate is the main responsible of neutralizing the polymer charge, while increasing the availability of salt from the solution to 10 mM results in chloride ions being the most abundant adsorbed counterions. When in contact with these relatively low ionic strength solutions, the film only adsorbs enough counter-ions to neutralize the electric charge of network segments. 22 This is because of the high cost that results from the loss of translational entropy when these counter-ions are confined inside the film. In this context, the adsorption behavior of Fig. 4B can be interpreted by considering panel C that shows the average, local net charge of glyphosate. A few nanometers above the top film surface, the average charge of the molecule becomes that of the bulk solution; most glyphosate molecules are in the −1 charge state, since its average charge is close to that value. Inside the film, however, glyphosate charge is around −2. Under the conditions of Fig. 4, adsorbing one glyphosate molecule brings more counter ion charge to the film for the entropic price of confining just one molecule. This explains

12


Page 12 of 28

Page 13 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

why the film adsorbs glyphosate which, in the solution, is much less abundant than chloride ions and similarly charged. As mentioned, increasing salt concentration lowers the entropic cost of confining such ions, which displaces the balance to favor chloride adsorption, as seen in Fig. 4B. The reason for glyphosate charge regulation can be explained considering panel D of Fig. 4, which shows the local pH. Sufficiently far from the film z >> h ≈ 130 nm, local pH takes the bulk value. Inside the film, however, a much higher pH establishes; local pH increases 2 to 3 units in the particular cases of Fig. 4. Upon adsorption, the higherpH environment results in the deprotonation of glyphosate acidic units, which explains the more negative charge of the molecule inside the film. This increase in pH not only favors adsorption but has also the additional advantage of potentially enhancing biodegradation of the pesticide inside the film. It has been reported that non neutral pH slows down microbial activity lowering the efficiency in glyphosate biodegradation. 20 In addition, local pH is relevant because this single quantity provides information on the local state of charge of all ionizable species, including the different units of glyphosate and those of the polymer network. Panel A of Fig. 5 shows the film pH, pHpol , calculated as an average from z = 0 up to z = h, as a function of the bulk pH. Film pH, which depends on the solution salt concentration, is higher than the pH imposed to the bulk solution, in almost all the scale. One of the consequences of this last result, combined with those shown in Fig. 3B, is that PAH films offer a wide range of conditions that can simultaneously drive adsorption and result in more favorable conditions for degradation inside the film. Because pH is higher inside the film, the average charge of adsorbed glyphosate is more negative, which is shown in Fig. 5B. This behavior depends on the salinity of the solution, the lower the salt concentration the more negatively charged the adsorbate is. The maximum deviation from bulk solution conditions occurs near pH 5, the conditions also shown in Fig. 4. As we have briefly mentioned, the more negative charge of glyphosate inside the film is due to deprotonation of its acidic units inside the film resulting from the higher pH environ-

13


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5: Plots showing the pH-dependence of different quantities at two salt concentrations, 1 mM (black-line curves) and 10 mM (red-line curves), and [glp] = 10 µM. A: average pH inside the film, pHpol (dashed-line curve corresponds to pHpol = pH); dotted lines show pHpol for solutions containing no glyphosate. B: Average net charge of adsorbed glyphosate molecules (dashed-line curve shows the solution charge); C: average degree of charge of phosphonate groups corresponding to adsorbed glyphosate molecules (dashed-line curve is that of solution molecules); D: average degree of protonation of network segments for solutions with (solid line) and without (dotted line) glyphosate (dashed-line curve represents the protonation of the isolated monomer in dilute solution).

14


Page 14 of 28

Page 15 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

ment. To illustrate this behavior, Fig. 5C shows the degree of charge of phosphonate units corresponding to adsorbed glyphosate molecules as a function of bulk pH. Compared to the bulk solution, inside the film this unit is significantly more deprotonated at the same pH. At pH 5, for example, the phosphonate unit is mostly protonated (uncharged) in the solution, while inside the film it is fully deprotonated (charged) at both salt conditions shown in the plot. The magnitude of this displacement toward deprotonation, however, does depend on the salt concentration, since the film pH also shows this dependence (see Fig. 5A). The higher pH inside the film also results in the deprotonation of PAH units; this behavior is presented in Fig. 5D. Namely, compared to the isolated monomer, these units are significantly more deprotonated (less charged) when confined to the polymer network, which depends on the salt concentration. In the absence of glyphosate, this behavior occurs to reduce electrostatic repulsions between like-charged polymer units. When glyphosate is in the bulk solution, this allows the polymer to be more protonated, although protonation is still much lower than ideal behavior (see Fig. 5D), particularly at low salt concentration. In this context, knowing the film pH provides information about the state of charge of all species inside the film. For example, using the intrinsic pKa of its units one can construct Fig. 2 and evaluate it at the film pH (rather than the bulk pH), which gives a reasonable approximation of the molecule charge inside the film. However, film pH results from the non-trivial balance between all of the physicochemical contributions to the free energy. Let us consider the following situation: a higher film pH allows for the adsorbate to be more strongly charged inside the hydrogel than in the solution, favoring adsorption due to stronger electrostatic attractions. This higher film pH, however, results in the deprotonation of network units lowering their degree of charge, which weakens the electrostatic attractions and disfavors adsorption. For both species, the chemical free energy cost of deprotonation favors a lower pH, equal to the bulk value. Therefore, the pH that establishes inside the film results from the conditions that yield thermodynamic equilibrium; such conditions emerge from the complex local interplay between all physical interactions, chemical equilibrium and

15


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

molecular organization at the environmental conditions imposed externally (bulk solution composition). Experimental access to this quantity requires very dilute concentrations of the probe; we have shown in Fig. 5A that adsorption, even at low bulk concentrations ([glp] = 10 µM), can modify the pH inside the material.

Figure 6: (A) Polymer volume fraction as a function of the distance to the supporting surface for three different networks at pH 5. (B) Glyphosate adsorption as a function of pH for the three different polymer films of panel A; inset shows the pH inside the film. In both panels the conditions are [salt] = 1 mM and [glp] = 10 µM.

We now investigate the effect of polymer density on the adsorption of glyphosate to PAH hydrogel films. To address this question we have considered three films with the same topology but having different polymer density. The volume fraction of these networks is shown at in Figure 6A as a function of the distance to the supporting surface. Figure 6B shows the pH dependence of glyphosate adsorption to these hydrogel films. Although the 16


Page 16 of 28

Page 17 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

films have similar thickness, increasing the polymer density enhances adsorption per unit area. This enhancement is due to a higher negative charge density that occurs in the films as the density of PAH increases. The degree of protonation, however, decreases as the volume fraction increases, effect that is compensated by the higher density of protonable PAH segments (see SI). Moreover, given the solution pH, Fig. 6B inset shows that the film pH changes with network density. As the polymer density increases, so does the pH inside the material. In particular, for the cases presented in Fig. 6, pHpol 7 occurs within the range of pH values 3.5 − 5. This behavior suggests that polymer density can be used as a design parameter not only to enhance adsorption but also to achieve optimal degradation conditions inside the material.

Figure 7: Adsorption isotherms: Plot of Γ as a function of solution concentration of glyphosate. The various curves correspond to different pH values. The salt concentrations of panel A and B are 1 mM and 10 mM, respectively.

Next, we consider how glyphosate adsorption depends on its bulk concentration. Adsorption isotherms are presented in Fig. 7 for different pH values and salt concentrations. Here, we see again that the effect of increasing salt concentration is reducing the effective capacity of the hydrogel to adsorb glyphosate. Moreover, the pH of maximum adsorption depends sensibly on glyphosate concentration as well as salt concentration. In the Supporting Information, we compare the results of Fig. 7, obtained with our molecular theory, with 17


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

well known, simple adsorption isotherm models. AMPA is the most abundant degradation product of glyphosate. Thus, in Fig. 8 we consider the adsorption of AMPA to the PAH film in terms of the composition of the bulk solution. These curves show the same general trends of the adsorption of glyphosate seen in Fig. 3. AMPA adsorption decreases with increasing salt concentration due to the enhancing role of chloride ions to neutralize the network charge (see Fig. 8B). Moreover, adsorption of AMPA is a non-monotonic function of the bulk pH (see Fig. 8B). At low pH, AMPA in only weakly charged (see Fig. 2), while the polymer is weakly charged at high pH. Under these conditions, no significant adsorption occurs. At intermediate pH, however, both molecules are strongly charged leading to the maximum in the pH-dependent adsorption profiles of Fig. 8B.

Figure 8: Plot of AMPA adsorption, Γ, as a function of the bulk solution composition: Panel A shows Γ as a function of the salt concentration for different pH values; panel B shows the adsorption as a function of the pH for various salt concentrations. The concentration of the adsorbate in the bulk solution is [ampa] = 10 µM.

The adsorption of glyphosate and that of AMPA are qualitatively similar. However, the adsorption of glyphosate is roughly two orders of magnitud larger than that of AMPA at the same conditions (compare Fig. 3 and Fig. 8). The reason for this behavior is the additional carboxylate group that glyphosate bears. Under most conditions, this group is deprotonated when glyphosate adsorbs, which means that this molecule bears one more negative charge inside the hydrogel (see SI). 18


Page 18 of 28

Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 9: Adsorption from mixtures of glyphosate and AMPA as a function of the pH, for solutions having [ampa] = 10−4 M (panel A) or [ampa] = 10−3 M (panel B). In both panels, [glp] = 10−5 M and [salt] = 1 mM. Dotted-line curves show the adsorption from pure solutions at otherwise the same conditions.

In this context, the next question that we address is: how is the adsorption from mixtures of glyphosate and AMPA? Figure 9 shows the adsorption from such mixtures as a function of pH at relatively low salt conditions. In panel A, the bulk concentration of AMPA is one order of magnitude larger than that of glyphosate. Under these conditions, the adsorption of glyphosate from pure solutions is more than an order of magnitude larger than the adsorption of AMPA from pure solutions. As a consequence, the adsorption of glyphosate from mixtures is not affected by the presence of AMPA. Contrarily, glyphosate reduces the adsorption of AMPA to approximately half of the value for pure solutions (in the pH range around

19


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

maximum adsorption). When increasing the bulk concentration of AMPA, the situation is qualitatively similar, as seen in Fig. 9B. Although the adsorption of glyphosate in the mixture diminishes respect to pure solutions, the main effect is on the adsorption of AMPA. Under these conditions that are a priori more favorable for AMPA adsorption, glyphosate significantly reduces the adsorption of AMPA when comparing mixtures to pure solutions.

Conclusion In this work, we have applied a molecular level theory to study the equilibrium adsorption to grafted PAH hydrogel films from salt solutions containing glyphosate, AMPA or both. Our molecular model includes description of size, shape, conformation and charge state of all chemical species, including the adsorbates and the polymer network. We have focused our attention in the role that solution composition (pH, salt and adsorbate concentration) has on the adsorption behavior. Adsorption of glyphosate (or AMPA) is a non-monotonic function of the solution pH, which is explained in terms of the charging behavior of both the polymer network and adsorbate. The pH of maximum adsorption depends on the salt and adsorbate concentration. Our predictions show that in finding the optimal conditions for adsorption, salt concentration is a critical variable. The lower the salinity of the medium, the more glyphosate (or AMPA) adsorbs to the film. In particular, a sufficiently low salt concentration allows for a wide range of pH values where adsorption is significant. Therefore, the optimal conditions for adsorption are achieved lowering the salt concentration. The reason for this behavior is the competitive adsorption between glyphosate and salt anions. These ions adsorb to the film to neutralize the charge in the polymer. Counterion confinement has an entropic cost associated with the loss of translational entropy. As a counter-ion, glyphosate is only slightly better than a monovalent anion: depending on the pH, this herbicide bears an additional negative charge, which favors its adsorption. Such adsorption brings the same counter-ion charge to film, but at a lower entropic cost of molec-

20


Page 20 of 28

Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

ular confinement. However, glyphosate adsorption has also some disadvantages respect to salt anion adsorption: first it is a larger molecule; and second, the additional charge require some of its titratable groups to be deprotonated, which increases the chemical free energy that describes this acid-base equilibrium. As a result, the ratio between glyphosate and salt concentrations becomes a critical factor in deciding the winner in this competitive adsorption game. Inside the film, a higher pH establishes than the bulk value. The main effect of this phenomenon is that the adsorbate regulates its charge. Upon adsorption, glyphosate is more negatively charged than in the bulk solution, which enhances electrostatic attractions with the positively charged network and drives adsorption. Equally important, this increase in pH has implications for glyphosate biodegradation, since microbial activity depends on the acidity of the medium. Therefore, PAH films offer an additional advantage to pesticide sequestration from aqueos solutions. Namely, under some experimental conditions biodegradation inside the material can be enhanced. AMPA is glyphosate main metabolite. In pure solutions, the adsorption of glyphosate is significantly higher than that of AMPA at the same conditions. This occurs because glyphosate bears an additional carboxylate group that deprotonates as the herbicide adsorbs. For this reason, glyphosate can hinder the adsorption of AMPA from binary mixtures. This behavior points to an additional advantage of PAH films for glyphosate sequestration and biodegradation. While biodegradation occurs inside this material glyphosate will remain adsorbed. Finally, in devising PAH films for glyphosate sequestration and degradation the material density of polymer is an important design variable. We have shown that polymer networks with high density of titratable units can enhance adsorption. In addition, the film pH can be modified using the network density if the goal is to optimize in-situ microbial degradation.

21


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Supporting Information Available Additional results. Adsorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgement This work was supported by CONICET and ANPCyT (PICT-2014-3377), Argentina. N. A. P. C. acknowledges a ANPCyT fellowship (PICT-2015-3425).

References (1) Woodburn, A. T. Glyphosate: Production, pricing and use worldwide. Pest Manag. Sci. 2000, 56, 309–312. (2) Benbrook, C. M. Trends in glyphosate herbicide use in the United States and globally. Environ. Sci. Eur. 2016, 28, 1–15. (3) Primost, J. E.; Marino, D. J.; Aparicio, V. C.; Costa, J. L.; Carriquiriborde, P. Glyphosate and AMPA, pseudo-persistent pollutants under real-world agricultural management practices in the Mesopotamic Pampas agroecosystem, Argentina. Environ. Pollut. 2017, 229, 771–779. (4) Martinez, D. A.; Loening, U. E.; Graham, M. C. Impacts of glyphosate-based herbicides on disease resistance and health of crops: a review. Environ. Sci. Eur. 2018, 30, 2. (5) Ronco, A. E.; Marino, D. J.; Abelando, M.; Almada, P.; Apartin, C. D. Water quality of the main tributaries of the Paraná Basin: Glyphosate and AMPA in surface water and bottom sediments. Environ. Monit. Assess. 2016, 188, 458.

22


Page 22 of 28

Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(6) Rendon-von Osten, J.; Dzul-Caamal, R. Glyphosate residues in groundwater, drinking water and urine of subsistence farmers from intensive agriculture localities: A survey in Hopelchén, Campeche, Mexico. Int. J. Environ. Res. Public Health 2017, 14, 595. (7) Castro Berman, M.; Marino, D. J.; Quiroga, M. V.; Zagarese, H. Occurrence and levels of glyphosate and AMPA in shallow lakes from the Pampean and Patagonian regions of Argentina. Chemosphere 2018, 200, 513–522. (8) Younes, M.; Galal-Gorchev, H. Pesticides in drinking water-A case study. Food Chem. Toxicol. 2000, 38, S87–S90. (9) World Health Organization, Glyphosate and AMPA in drinking-water ; 2005. (10) Liu, Z.; Zhu, M.; Yu, P.; Xu, Y.; Zhao, X. Pretreatment of membrane separation of glyphosate mother liquor using a precipitation method. Desalination 2013, 313, 140– 144. (11) Lan, H.; He, W.; Wang, A.; Liu, R.; Liu, H.; Qu, J.; Huang, C. P. An activated carbon fiber cathode for the degradation of glyphosate in aqueous solutions by the ElectroFenton mode: Optimal operational conditions and the deposition of iron on cathode on electrode reusability. Water Res. 2016, 105, 575–582. (12) Xing, B.; Chen, H.; Zhang, X. Efficient degradation of organic phosphorus in glyphosate wastewater by catalytic wet oxidation using modified activated carbon as a catalyst. Environ. Technol. 2018, 39, 749–758. (13) Song, J.; Li, X. M.; Figoli, A.; Huang, H.; Pan, C.; He, T.; Jiang, B. Composite hollow fiber nanofiltration membranes for recovery of glyphosate from saline wastewater. Water Res. 2013, 47, 2065–2074. (14) Loperena, L.; Ferrari, M. D.; Saravia, V.; Murro, D.; Lima, C.; Ferrando, L.; Fernández, A.; Lareo, C. Performance of a commercial inoculum for the aerobic 23


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

biodegradation of a high fat content dairy wastewater. Bioresour. Technol. 2007, 98, 1045–1051. (15) Zhou, C.; Jia, D.; Liu, M.; Liu, X.; Li, C. Removal of glyphosate from aqueous solution using nanosized copper hydroxide modified resin: Equilibrium isotherms and kinetics. J. Chem. Eng. Data 2017, 62, 3585–3592. (16) Santos, T. R. T.; Andrade, M. B.; Silva, M. F.; Bergamasco, R.; Hamoudi, S. Development of α- and γ-Fe2O3 decorated graphene oxides for glyphosate removal from water. Environ. Technol. 2017, 3330, 1–20. (17) Guo, L.; Cao, Y.; Jin, K.; Han, L.; Li, G.; Liu, J.; Ma, S. Adsorption characteristics of glyphosate on cross-linked amino-starch. J. Chem. Eng. Data 2018, 63, 422–428. (18) Zavareh, S.; Farrokhzad, Z.; Darvishi, F. Modification of zeolite 4A for use as an adsorbent for glyphosate and as an antibacterial agent for water. Ecotoxicol. Environ. Saf. 2018, 155, 1–8. (19) Hallas, L. E.; Adams, W. J.; Heitkamp, M. A. Glyphosate degradation by immobilized bacteria: Field studies with industrial wastewater effluent. Appl. Environ. Microbiol. 1992, 58, 1215–1219. (20) la Cecilia, D.; Maggi, F. Analysis of glyphosate degradation in a soil microcosm. Environ. Pollut. 2018, 233, 201–207. (21) Longo, G. S.; Olvera de la Cruz, M.; Szleifer, I. Equilibrium adsorption of hexahistidine on pH-responsive hydrogel nanofilms. Langmuir 2014, 30, 15335–15344. (22) Longo, G. S.; Szleifer, I. Adsorption and protonation of peptides and proteins in pH responsive gels. J. Phys. D. Appl. Phys. 2016, 49, 323001. (23) Hagemann, A.; Giussi, J. M.; Longo, G. S. The use of pH gradients in responsive polymer hydrogels for the separation and localization of proteins from binary mixtures. 24


Page 24 of 28

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(24) Nap, R.; Gong, P.; Szleifer, I. Weak polyelectrolytes tethered to surfaces: Effect of geometry, acid-base equilibrium and electrical permittivity. J. Polym. Sci. Part B Polym. Phys. 2006, 44, 2638–2662. (25) Gong, P.; Genzer, J.; Szleifer, I. Phase behavior and charge regulation of weak polyelectrolyte grafted layers. Phys. Rev. Lett. 2007, 98, 018302. (26) Kohn, W.; Sham, L. J. Self-consistent equations including exchange and correlation effects. Phys. Rev. 1965, 140, 1133–1138. (27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision B.03. Gaussian, Inc., Pittsburgh PA, 2003. (28) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B 1988, 37, 785–789.

25


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(29) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648–5652. (30) Sadi, B. B. M.; Vonderheide, A. P.; Caruso, J. A. Analysis of phosphorus herbicides by ion-pairing reversed-phase liquid chromatography coupled to inductively coupled plasma mass spectrometry with octapole reaction cell. J. Chromatogr. A 2004, 1050, 95–101. (31) Bhatia, S. R.; Khattak, S. F.; Roberts, S. C. Polyelectrolytes for cell encapsulation. Curr. Opin. Colloid Interface Sci. 2005, 10, 45–51. (32) Chen, Z.; He, W.; Beer, M.; Megharaj, M.; Naidu, R. Speciation of glyphosate, phosphate and aminomethylphosphonic acid in soil extracts by ion chromatography with inductively coupled plasma mass spectrometry with an octopole reaction system. Talanta 2009, 78, 852–856. (33) Lawrence, P. G.; Lapitsky, Y. Ionically cross-linked poly(allylamine) as a stimulusresponsive underwater adhesive: Ionic strength and pH effects. Langmuir 2015, 31, 1564–1574. (34) Mann, B. A.; Holm, C.; Kremer, K. Swelling of polyelectrolyte networks. J. Chem. Phys. 2005, 122, 154903. (35) Quesada-Pérez, M.; Maroto-Centeno, J. A.; Mart´ın-Molina, A. Effect of the counterion valence on the behavior of thermo-sensitive gels and microgels: A Monte Carlo simulation study. Macromolecules 2012, 45, 8872–8879. (36) Koˇsovan, P.; Richter, T.; Holm, C. Modeling of polyelectrolyte gels in equilibrium with salt solutions. Macromolecules 2015, 48, 7698–7708. (37) Hofzumahaus, C.; Hebbeker, P.; Schneider, S. Monte Carlo simulations of weak polyelec-

26


Page 26 of 28

Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

trolyte microgels: pH-dependence of conformation and ionization. Soft Matter 2018, 14, 4087–4100. (38) Berendsen, H. J. C.; van der Spoel, D.; van Drunen, R. GROMACS: A message-passing parallel molecular dynamics implementation. Comput. Phys. Commun. 1995, 91, 43– 56. (39) van der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A. E.; Berendsen, H. J. C. GROMACS: Fast, flexible, and free. J. Comput. Chem. 2005, 26, 1701–1718. (40) Abraham, M. J.; Murtola, T.; Schulz, R.; Páll, S.; Smith, J. C.; Hess, B.; Lindahl, E. GROMACS: High performance molecular simulations through multi-level parallelism from laptops to supercomputers. SoftwareX 2015, 1-2, 19–25. (41) Kremer, K.; Grest, G. S. Dynamics of entangled linear polymer melts: A moleculardynamics simulation. J. Chem. Phys. 1990, 92, 5057–5086. (42) Longo, G. S.; Olvera de la Cruz, M.; Szleifer, I. Molecular theory of weak polyelectrolyte gels: The role of pH and salt concentration. Macromolecules 2011, 44, 147–158.

27


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

For Table of Contents Use Only

28


Page 28 of 28

Using Polymer Hydrogels for Glyphosate Sequestration from Aqueous

Recommend Documents